Viscoelastic behavior of the cell wall of exfoliated graphite

The cell wall (∼60-graphite-layer thick, average) in the cellular structure of exfoliated graphite compacts is viscoelastic, due to the shear between graphite layers. The viscous character decreases with increasing solid content (volume fraction), due to the increasing difficulty of shear, which becomes limited at solid contents above 4 vol.%. The elastic character is essentially independent of the solid content, weakening slightly with increasing solid content. The viscous character is stronger under in-plane flexure than compression in the compaction direction, due to the preferred orientation of the graphite layers. At the lowest solid content of 1.0 vol.%, the loss-tangent/solid-content is 35 and 25 under flexure and compression respectively, the storage-modulus/solid-content is 125 MPa and 46 kPa under flexure and compression respectively, and the loss-modulus/solid-content is 45 MPa and 13 kPa under flexure and compression respectively. The viscous character is strong under both flexure and compression, whereas the elastic character is much stronger under flexure than compression. The loss-tangent/solid-content decreases with increasing solid content, leveling off at 0.9 at 15 vol.% solid. The loss-modulus/solid-content also decreases with increasing solid content. The highest values of the loss-tangent/solid-content, storage-modulus/solid-content and loss-modulus/solid-content are greater for exfoliated graphite compacts than rubber or carbon black compacts.